1. Field of the Invention
The invention relates to the manufacture of composite structures by spraying multiple layers of polyurethane onto a mold or substrate, and to compositions suitable for use therein. The invention further relates to resin transfer molding processes employing the compositions of the invention, and to products prepared thereby.
2. Description of the Related Art
Spray applied polymer systems have very widespread use in preparing composite structures, for example bathtubs, spas, shower enclosures, boat hulls, storage tanks, and the like. In these applications, addition curable resins such as unsaturated polyester and vinyl ester resins are commonly used. Epoxy resins are sometimes used in demanding applications, but suffer the disadvantage of relatively high cost. The resins used in the largest volume commercially are unsaturated polyester resins. The latter resins also contain considerable amounts of styrene which serves both as a comonomer and diluent.
The resin systems are typically combined with glass fiber reinforcement, which may be woven or non-woven, or present as chopped strand. Typically the spray applied resin is handworked into the fiberglass. This method is especially useful for preparing boat hulls, for example.
A principle drawback of unsaturated polyester resins is that styrene monomer is listed as a class 1 carcinogen, and its use is becoming increasingly regulated. Spray application exacerbates these problems since a fine mist is invariably produced in the spray process, from which styrene rapidly volatilizes. Workers must generally wear protective breathing devices, and enclosed spaces must be carefully ventilated.
Polyurethanes have occasionally been used in spray applications, mostly in the field of rigid insulating foam. Elastomeric foams have also been used in sandwich structures, for example between fiber reinforced polyester layers. Polyurethane systems are at least two component systems where the isocyanate-reactive components such as polyols, crosslinkers, chain extenders, and the like, in addition to catalysts are stably prepared as a “B-side,” and the isocyanate(s) are contained into the “A-side.” The A and B sides are supplied to a mixhead and intensively mixed; both static and mechanical mixers as well as impingement mixing have been used. Less commonly in spray applications, individual components, perhaps as many as 6 or 7 components, are supplied to the mixhead rather than A and B sides. The mixhead in such applications becomes very unwieldy, and such systems are generally limited to foam-in-place applications such as for seating foams and slab foams, and in RIM (reaction injection molding). Following mixing of the isocyanate and isocyanate-reactive components, rapid reaction occurs, producing the polyurethane polymer.
Polyurethanes have numerous advantageous properties as compared with unsaturated polyester resins, and as they contain no styrene, their use eliminates that concern from manufacturing operations. Unfortunately, the cost of polyurethane systems is somewhat higher than polyester systems. More importantly, while tensile elongation may be superior to cured polyester, modulus is generally somewhat inferior. Many structures which are desired to be spray manufactured require high stiffness. Heat distortion temperature is also an important parameter in many applications. Flexural modulus of sprayed polyurethane systems have been invariably below 600,000-700,000 psi, which is too low for many demanding applications.
Adding fibrous or particulate fillers is one method of increasing modulus. However, chopped fibers cannot ordinarily be incorporated into the reactive components themselves, but are often supplied to the spray cone, which directs the then-coated fibers to the substrate. Particulate fillers must be of such size so as to remain sprayable, which generally means that only fillers of very small size and correspondingly high surface area must be used. However, when appreciable amounts of high surface area filler are added to the polyol side (B-side), the viscosity increases greatly in proportion to filler content, such that at high filler loadings, the composition cannot be efficiently conveyed to the spray head or be sprayed. Thus, the highest amount of filler tolerable in the polyol side is approximately 50% by weight. Fillers are not generally added to the isocyanate (A-side), and when preparing laminate structures with multiple layers of polyurethane, use of fillers has been avoided due to concerns with interlaminar adhesion.
If filler could also be added to the isocyanate side (A-side) as well, the total amount of filler in the cured system would be able to be increased. In the past, fillers have only been added to the isocyanate side for molding and casting operations by incorporating the fillers immediately prior to use. An example of the latter is talc which, when added to non-sprayable polyurethane systems along with glass flakes, can be used to form a non-sagging putty-like mixture useful for repairing bumpers and fascias of automobiles, as disclosed in U.S. Pat. No. 5,607,998. These mixtures are clearly not sprayable.
However, in liquid polyurethane systems, even talc has been considered too reactive for incorporation into the isocyanate side of the polymer system, as surface hydroxyl groups would be expected to react with the isocyanate, and thus the viscosity of the A-side would increase rapidly during transportation and storage. Numerous fillers have been proposed for incorporation into the B-side, but have been considered non-reactive in the overall system, and thus are stated to be incapable of providing sufficient reinforcement to the matrix, preventing high modulus products from being obtained. Thus, for example, in U.S. Pat. No. 5,693,696, sand, clay, and talc are all disclosed as potential fillers for addition to the polyol side (B-side), but must be treated with an adhesion promoter which reacts with surface hydroxyl groups on the filler and also bears an isocyanate-reactive group. Aminoalkyltrialkoxysilanes are touted for this purpose, the alkoxy groups covalently bonding to the filler surface hydroxyl groups, leaving a very reactive alkylamino group to react with the isocyanate. Use of such reactive adhesion promoters adds additional process steps and expense.
U.S. Pat. No. 6,211,259 B1 discloses the use of fillers such as clay, talc, and alumina trihydrate in the polyol side of a polyurethane system which may be sprayed. However, it is difficult to incorporate high amounts of fillers in such systems. U.S. Pat. No. 6,881,764 indicates that fillers are added to the B-side (resin side) of polyurethane systems, and employs glass cullet as a filler. It must be remembered, that the filler content of the polyol side is “diluted” by the A-side upon mixing, and thus a polyol filler content of, for example, 50 percent by weight becomes only 25 percent by weight in the cured product in conventional 1:1 mix ratios.
As disclosed in the above references, particularly U.S. Pat. No. 5,693,696, active hydrogen-containing fillers have been described as not being well incorporated into polyurethanes unless first rendered hydrophobic, or functionalized with organic groups which are also reactive with isocyanates.
It is further desired that the composite structures be impact resistant. Both polyester and epoxy resin systems tend to produce fiber reinforced products which, while displaying high flexural modulus and tensile strength, are nevertheless quite brittle, as indicated by relatively low impact resistance. During manufacturing, for example, the impact of a fall from a transport dolly or the like is sufficient to generate cracks which render the article unusable. It would be desired to produce articles which do not manifest such proclivity to impact damage and yet which exhibit acceptable tensile strength and modulus.
Surprisingly, adding filler in the form of chopped glass fibers to polyurethane systems does not solve these problems. At high loadings of glass fibers, impact strength is adequate, but flexural modulus and tensile strength are low. Surprisingly, an increase in fiber content causes these properties to worsen rather than improve. U.S. Pat. No. 4,543,366 discloses adding particulate and/or chopped fiber fillers up to a total amount of 30 weight percent based on the weight of the urethane system. However, these amounts of fillers are inadequate to produce articles which simultaneously offer high tensile strength, high flexural modulus, resistance to impact damage, and satisfactory heat distortion temperature. Thus, in the twenty plus years since the U.S. Pat. No. 4,543,366 issued, polyurethane systems were not able to supplant polyester systems.
It would be desirable to provide polyurethane systems which are sprayable and which yet contain more than 30 weight percent of filler. It would further be desirable to employ fillers in their unmodified form, i.e. not having been functionalized with isocyanate-reactive surface groups, to simultaneously provide multilayer laminates of good interlaminar adhesion, high tensile strength and flexural modulus, high resistance to impact damage, and high hardness.
Structural parts have also been made by processes generally termed resin transfer molding, or “RTM”. There are numerous variants of such processes, such as vacuum assisted RTM, or “VARTM”. All these variants are termed “RTM” herein unless specified otherwise.
Resin transfer molding is a closed mold, low pressure molding process, sometimes referred to as a liquid molding process, applicable to the fabrication of complex high performance composite articles of both large and small size. Several different resin transfer molding processes are well known to the skilled of the art. The process is differentiated from various other molding processes in that a reinforcement material, such as fiberglass or other fiber reinforcement material, is first placed into a molding tool cavity and then combined with resin within the mold cavity to form a fiber reinforced plastic (“FRP”) composite product.
Typically, a pre-shaped fiber reinforcement, sometimes referred to as a reinforcement preform, is positioned within a molding tool cavity and the molding tool is then closed. A feed line connects the closed molding tool cavity with a supply of liquid resin and the resin is pumped or “transferred” into the tool cavity where it impregnates and envelops the fiber reinforcement and subsequently cures. The cured or semi-cured FRP product then is removed from the molding tool cavity. As used herein, the terms resin transfer molding and RTM are used to refer generically to molding processes wherein fiber reinforcement is positioned in a molding tool cavity into which resin is subsequently introduced. Thus, variations such as so-called press molding or squeeze molding, structural reaction injection molding (“SRIM”) and the like are within the scope of such terms. Structural reaction injection molding uses a highly reactive resin system comprising two components pumped from separate holding tanks under pressure into an impingement mixing chamber and from there into the molding tool cavity. The tooling typically comprises a metallic shell to facilitate heat transfer. Although the mixing pressure is relatively high, the overall pressure of the resin in the molding tool typically is only about 50-100 psi. The resin flows into the molding tool cavity and wets-out the fiber reinforcement as the curing reaction is occurring. Typically, the fiber reinforcement material can be used in amounts up to about 20-30/weight percent of the fiber plus resin composite. Due to rapid resin cure, flow distances may be limited and for longer flow distances multiple inlet ports may be required.
Another variant of resin transfer molding, referred to generally as high speed resin transfer molding, is particularly suitable for commercial production of products requiring a three dimensional reinforcement preform. Fiber content typically is in the 35-50 weight percent range. Tooling for high production volumes typically is made of steel in order to contain molding pressures of 100-500 psi and for good heat transfer characteristics. For more limited production requirements, aluminum or zinc tooling may be acceptable. Typically, molding is carried out at elevated temperatures to reduce the cure time. The preform is positioned within the molding tool cavity, the mold is closed and resin is injected. At higher reinforcement levels, that is, at higher fiber weight content, the mold may be left slightly opened during resin injection to promote more rapid filling of the molding cavity; the mold cavity would then be fully closed. Preferably, the curing of the resin is accomplished in the mold such that the product will require no post-bake cycle and will have acceptable dimensional stability. For complex components or components having critical dimensional tolerance requirements, a fixtured post-cure may be required for adequate dimensional stability.
In view of the fact that RTM processes allow placement of fiber reinforcement materials, containing any of the various available fiber types or combinations thereof, in the mold cavity with minimal subsequent movement of the reinforcement preform during injection of the resin, the fiber reinforcement preform can be designed for optimum performance at minimum weight. That is, the fiber reinforcement preform can be designed and assembled with the most appropriate amount and type of reinforcement fiber (e.g., glass, graphite, aramid, etc., either chopped or continuous, random or oriented) in each portion of the preform. This yields a product of more optimum performance at reduced weight. Also, the low pressure required for resin injection often allows the use of less expensive presses and the use of tooling somewhat less costly than that employed in high pressure compression molding or thermoplastic stamping processes. Furthermore, there is the opportunity for significant assembly and tooling expense reduction where a significant degree of sub-part integration is achieved. That is, the RTM manufacture can integrate into a single, large, complex FRP component a number of sub-components which in metal would be manufactured separately and then assembled. In addition, the low pressures employed in RTM processes often enable larger structures to be produced than would be practical by other molding processes. Current compression molding processes, for example, are constrained by the cost and/or availability of sufficiently large presses.
Considerable effort is now being made to further advance the technology of RTM processes. Specifically, development is on-going in the areas of tooling fabrication, resin chemistry, control of resin flow and cure rates, and fabrication of complex preforms. With respect to fabrication of the preform, chopped, random fiber reinforcement material may be employed for its low cost and ease of use. One of the most versatile techniques for creating RTM-preforms, especially 3-dimensional preforms, is the so called spray-up process, wherein chopped glass roving or other chopped fiber reinforcement material is sprayed onto a forming mandrel from a chopper gun. Typically, the fibers are resin coated or a small amount of resin is introduced into the stream of chopped fibers to cause it to be retained on the screen. When the fibers accumulate to the proper weight or depth the resin can be cured to fix the shape of the resultant preform. Typically, the forming mandrel is a screen and vacuum is applied to the back of the screen to hold the fiber onto the screen as they accumulate and also to help ensure uniformity of fiber depth in the various areas of the screen. As the holes in the screen become covered by fiber, the remaining open areas tend to attract more fiber, causing a self-leveling action. This is capable of producing preforms of complex, near net shape with low waste.
A significant difficulty in the use of RTM processes, however, involves the fragile nature of the fiber reinforcement preforms. Preforms typically are handled and transported during manufacture and storage and during placement into the RTM molding tool cavity. Such handling and transport can cause damage, dislocation and loss of the reinforcement material of the preform. This can diminish the quality of the finished FRP product. Also, loose fibers can be a problem in the work area. In addition, when a preform is placed into a molding tool cavity, it must not extend beyond the desired seal or pinch off areas in the tool, since this could interfere with the mold closing and sealing properly. Particular care must be taken that the fibers of the reinforcement material do not extend from the preform into such areas or become dislodged and fall into such areas. This is a concern especially in the case of preforms, e.g. sprayed-up preforms as described above, in which chopped, randomly oriented fibers are employed. A covering is sometimes employed on a preform during shipment and handling, which covering is discarded prior to placement of the preform into the molding tool cavity. However, some reinforcement fibers may still be disrupted and lost during placement of the preform into the molding tool cavity, thus, allowing loose fibers interfering with the closure and sealing of the molding tool cavity.
A problem with polyurethane RTM is that despite the relatively high and uniform fiber content, obtaining products of high modulus, high tensile strength, and elevated heat distortion temperatures is still problematic. This may be due in part to the same problems discussed previously with respect to spray systems employing glass fibers, where matrix adhesion to the reinforcing fibers is still not optimal. Thus, it would be desirable to provide a polyurethane RTM system with higher mechanical properties than heretofore available.